Fuel cell system

ABSTRACT

A fuel cell system has a fuel cell ( 1 ) performing power generation as a result of reactions in supplied gases, a humidifying device ( 34 ) for humidifying at least one supplied gas by using water from water tank ( 31 ), and a coolant temperature regulation device ( 21, 22, 25, 26, 27, 28, 51 ) for regulating the temperature of the coolant flowing within the fuel cell ( 1 ) in order to control the temperature of the fuel cell ( 1 ), and a defrosting device ( 61 ). The defrosting device ( 61 ) melts ice in the water tank ( 31 ) during a startup operation of the fuel cell system by applying heat contained in the coolant to the ice. Here, the coolant has an increased temperature as a result of waste heat produced during power generation inside the fuel cell ( 1 ).

FIELD OF THE INVENTION

This invention relates to a fuel cell system and more particularly,relates to startup control for a fuel cell system.

BACKGROUND OF THE INVENTION

A polymer electrolyte fuel cell comprises a polymer electrolyte membranedisplaying proton permeability and a porous catalyst-electrode providedon both sides of the membrane. Air (or oxygen) and hydrogen arerespectively supplied to electrodes and power generation is performed asa result of electrochemical reactions between hydrogen and oxygenpresent in the air.

When protons pass through the polymer membrane, the polymer membranenear to the anode is dried as a result of the migration of protonstogether with water molecules due to electro-osmosis. This dryingprocess reduces the electrical conductivity of the polymer membrane andtherefore has an adverse effect on power generation characteristics. Thedrying process may be prevented by moisturizing the hydrogen gas and airwhich are supplied to the electrodes. Tokkai 2001-256989 published bythe Japanese Patent Office in 2001 discloses a humidifying device usingwater (or pure water) from a water tank in order to humidify suppliedgases such as hydrogen and air. A polymer electrolyte fuel cellemploying an external humidifying system is provided with a humidifyingdevice for humidifying gases supplied to the fuel cell on an externalsection of the fuel cell. However when the fuel cell system is disposedin an external environment at a temperature of less than 0° C., it isnot possible to humidify supplied gases such as hydrogen or air due tothe fact that the water in the water tank freezes. Consequently, thefuel cell can only be started up after melting ice in the water tank. Aprior art technique prevents freezing of water by using an electricalheater to heat the water tank or the humidifying device when theexternal temperature is less than 0° C.

SUMMARY OF THE INVENTION

However the technique above increases the load on the battery due to theextremely large amount of power used by the electrical heater in orderto prevent freezing of water.

A fuel cell system sometimes comprises a temperature regulation devicewhich regulates the temperature of a coolant flowing through the fuelcell so that the fuel cell is maintained to a suitable temperature. Thetemperature of the coolant in the fuel cell is increased using wasteheat resulting from power generation in the fuel cell.

It is therefore an object of this invention to obtain a temperatureincrease at startup of the fuel cell system so as to melt ice in thewater tank without extra fuel consumption or power consumption.

In order to achieve the above object, this invention provides a fuelcell system having: a fuel cell generating power as a result of chemicalreactions between supplied gases, wherein a coolant flows in the fuelcell and undergoes a temperature increase as a result of absorbing wasteheat produced by power generation in the fuel cell; a water tank; ahumidifying device for humidifying at least one supplied gas by usingwater from the water tank; and a coolant temperature regulation devicefor regulating a temperature of the coolant flowing inside the fuel cellso as to control the temperature of the fuel cell.

The fuel cell system comprises a defrosting device for melting ice inthe water tank by applying heat of the coolant to the water tank; acoolant recirculation passage for allowing a recirculation of thecoolant through the defrosting device and the fuel cell; a flowgenerator for generating a flow of the coolant from the fuel cell to thedefrosting device; and a controller for controlling a startup operationof the fuel cell system. The controller has the function of controllingthe flow generator to generate a flow of coolant from the fuel cell tothe defrosting device so as to melt ice in the water tank while thestartup operation of the fuel cell system. The coolant has the functionof absorbing waste heat resulting from power generation operations andmelting ice in the water tank.

Further, this invention provides a control method for controlling a fuelcell system, the fuel cell system having: a fuel cell generating poweras a result of chemical reactions between supplied gases, wherein acoolant flows in the fuel cell and undergoes a temperature increase as aresult of absorbing waste heat produced by power generation in the fuelcell; a water tank; a humidifying device for humidifying at least onesupplied gas by using water from the water tank; and a coolanttemperature regulation device for regulating a temperature of thecoolant flowing inside the fuel cell so as to control the temperature ofthe fuel cell.

The control method comprises the steps of providing a defrosting devicefor melting ice in the water tank by applying heat of the coolant to thewater tank; providing a coolant recirculation passage for allowing arecirculation of the coolant through the defrosting device and the fuelcell; and generating a flow of coolant from the fuel cell to thedefrosting device so as to melt ice in the water tank while a startupoperation of the fuel cell system.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system according to a firstembodiment of this invention.

FIG. 2 is a flowchart of a setting routine for a below-freezing startupmode flag as defined by the first embodiment.

FIG. 3 is a flowchart showing a first startup control routine accordingto the first embodiment.

FIG. 4 is a flowchart showing a second startup control routine accordingto the first embodiment.

FIG. 5A is a graph showing temporal variation of heat generation in acombustor and power generation of a fuel cell according to the firstembodiment. FIG. 5B is a graph showing the temperature of the fuel cellaccording to the first embodiment. FIG. 5C is a graph showing the amountof melted ice in a water tank according to the first embodiment.

FIG. 6 is a schematic diagram of a fuel cell system according to asecond embodiment.

FIG. 7 is a flowchart showing a startup control routine according to thesecond embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although fuel cell systems shown in the embodiments below are adaptedfor use with a vehicle, the fuel cell systems may be suitably modifiedin order to allow use in an electrical appliance.

Referring to FIG. 1 of the drawings, a fuel cell system as defined by afirst embodiment of the invention will be described.

The fuel cell system comprises a fuel cell 1 provided with an airpassage, a hydrogen passage and a coolant passage 95, and a humidifyingdevice 34 disposed on an external section of the fuel cell 1. The fuelcell 1 is a polymer electrolyte fuel cell. In a usual case, hydrogen gasand air supplied to the fuel cell 1 are humidified using water resultingfrom chemical reactions extracted by a porous separator in the fuel cell1. Hereafter supplied air and hydrogen gas are sometimes simply referredto as supplied gas. On the other hand, when the temperature in the fuelcell 1 increases as a result of an increase in the load on the fuel cell1 and thus a suitable humidity level can not be maintained by onlyhumidifying the inside of the fuel cell 1 (for example, in thetemperature region greater than or equal to a fixed value T3 in FIG. 5),the humidifying device 34 performs auxiliary humidifying operations onthe air and hydrogen gas using water from a water tank 31. Thehumidified air and hydrogen gas is supplied to each electrode in thefuel cell 1.

The hydrogen is supplied from a hydrogen tank 2 to an injector 5 througha hydrogen supply passage 6. The hydrogen is introduced from theinjector 5 into a hydrogen passage inlet 7 of the fuel cell 1. Ahydrogen passage outlet 8 of the fuel cell 1 is connected to theinjector 5 through a hydrogen recirculation passage 9. When a cut-offvalve 3 is in the open position, hydrogen returns from the hydrogenpassage outlet 8 to the injector 5 through a cut-off valve 70. Thecut-off valve 3 is normally in the closed position. Air is pressurizedby a compressor 11 and is supplied to an air passage inlet 13 of thefuel cell 1 through an air supply passage 12.

Water in the water tank 31 is transferred under pressure to the injector34 (humidifying device) through a water supply passage 33 from a watersupply pump 32. Water is injected into the hydrogen supply passage 6 andthe air supply passage 12 from the water injector 34. This operationallows the hydrogen gas and the air introduced into the fuel cell 1 tobe humidified.

A pressure regulator valve 35 is provided in the water supply passage 33in order to maintain the water supply pressure to a fixed value. A firstreturn passage 36 extending from the pressure regulation valve 35 isconnected to the upstream side of the water supply pump 32 on the watersupply passage 33. A cut-off valve 38 which is normally in the closedposition is provided in a second return passage 37 which branches fromthe water supply passage 33 downstream of the water pump 32 and returnswater to the water tank 31. A cut-off valve 40 which is normally in theclosed position is provided in a third return passage 39 which branchesfrom the first return passage 36 of the pressure regulator valve 35 andreturns water to the water tank 31. When the cut-off valves 38, 40 areboth opened, water is drained to the water tank 31 through the returnpassage 37, 39.

After high levels of power generation by the fuel cell 1 are enabled, acontroller 51 regulates a hydrogen flow amount flowing in the hydrogensupply passage 6 in response to the required power generation amount bycontrolling a pressure control valve 4 provided in the hydrogen supplypassage 6. Even when for example the required power generation amount isvaried, the controller 51 controls the air flow amount from thecompressor 11 in order to obtain an optimal ratio (fixed value) betweenthe flow amounts of air and hydrogen in response to signals from a flowamount sensor 52 for detecting a hydrogen flow amount and a flow amountsensor 53 for detecting an air flow amount in the air supply passage 12.

The controller 15 comprises a microcomputer having a central processingunit (CPU) for running programs, read-only memory (ROM) for storingprograms and data, random access memory (RAM) for temporarily storingdata acquired as computing results from the CPU, and an input/outputinterface (I/O interface).

The fuel cell system comprises a flow generator for generating a flow ofcoolant. The flow generator comprises a coolant pump 21 which isdisposed in a coolant recirculation passage and transfers coolant underpressure. The fuel cell system comprises a coolant temperatureregulation device which regulates the temperature of the coolant flowinginside the fuel cell 1 so that when high levels of power generation areenabled in the fuel cell 1, the temperature in the fuel cell 1 issubstantially fixed. The coolant temperature regulation device comprisesa coolant recirculation passage for recirculating coolant inside thefuel cell 1 and in a radiator 26, a first bypass passage 27 branchingfrom the coolant recirculation passage and bypassing the radiator 26,and a three-way valve (passage switching device) for switching thepassage for the coolant from a passage passing through the radiator 26to a passage passing through the first bypass passage 27. The controller51 uses the three-way valve 28 in order to selectively switch the twopassages in order to control the coolant temperature in the fuel cell.

The radiator 26 performs heat exchange using outside air entering whilethe vehicle is running. A pair of coolant passages 22, 25 connect acoolant passage 95 in the fuel cell and the radiator 26. Thus, thecoolant recirculation passage is provided with a radiator 26, a coolantpassage 95 inside the fuel cell 1, a coolant passage 22 and a coolantpassage 25. Coolant flowing in the coolant recirculation passage flowsfrom the outlet of the coolant pump 21 to the coolant passage 22, thecoolant passage 95 in the fuel cell 1, the coolant passage 25 and theradiator 26. Thereafter the coolant returns to the inlet of the coolantpump 21.

The first bypass passage 27 bypassing the radiator 26 branches from thecoolant passage 25 downstream of the fuel cell 1. The first bypasspassage 27 is connected with the coolant passage 22 through thethree-way valve 28 (passage switching device).

When no current is applied, the three-way valve 28 connects the port Awith the port C and isolates the port B from the port C. When current isapplied, the three-way valve 28 cuts off the connection between the portA and the port C and connects the port B with the port C. Here thecurrent serves as a command signal. The passage for coolant is switchedfrom a passage passing through the radiator 26 to a passage passingthrough the first bypass passage 27 as a result of activation ornon-activation of the three-way valve 28.

After high levels of power generation in the fuel cell 1 are enabled,the controller 51 controls the three-way valve 28 so that thetemperature of coolant in the fuel cell 1 coincides with a referencetemperature (substantially fixed temperature). In other words, thecontroller 51 applies or does not apply a current to the three-way valve28 and thus commands the three-way valve 28 to selectively switch theflow of coolant through the radiator 26 when the coolant temperature ishigher than the reference temperature and through the first bypasstemperature 27 when the coolant temperature is lower than the referencetemperature. The reference temperature is determined in advancedepending on the state of the fuel cell. Additionally the controller 51controls the coolant flow amount injected by the coolant pump 21 inresponse to a signal from a temperature sensor 83 for detecting acoolant temperature in the coolant passage outlet 24, a pressure sensor55 for detecting a coolant pressure and a temperature sensor 54 fordetecting coolant in the coolant passage inlet 23. The temperaturesensor 83 detects a coolant temperature in the coolant passage outlet 24as a measure of the temperature of the fuel cell 1. However instead ofthe coolant temperature in the coolant passage outlet 24, thetemperature sensor 83 may detect directly the temperature of the fuelcell 1.

The electrical power generated by the fuel cell 1 is supplied toelectrical apparatuses such as a battery or an electrical drive motor.When the vehicle mounting the fuel cell system is parked and left in anexternal temperature of less than or equal to 0° C., water in the watertank 31 freezes and supplied hydrogen gas and air can not be humidified.Consequently in this embodiment, a defrosting device or a heater isprovided in order to melt ice in the water tank 31 using heat from thecoolant which has an increased temperature using waste heat produced bypower generation within the fuel cell 1.

The water tank 31 is disposed downstream of the fuel cell 1 relative tothe flow of coolant. A section of the coolant passage 25 which isconnected to the coolant passage outlet 24 of the fuel cell 1 isprovided in the water in the water tank 31. The section of the coolantpassage in the water tank 31 is formed in the shape of a coil forexample and serves as a heat exchanging section 61 (heat exchanger)allowing heat exchange between the coolant and the water in the watertank 31. In other words, the heat exchange section 61 is the defrostingdevice.

A heat exchanger 65 is provided along the first bypass passage 27 andperforms heat exchange operations between the coolant in the firstbypass passage 27 and combustion gas. The heat exchanger 65 isintegrated with an electrically heated catalyst 67 (EHC) and a catalyticcombustor 66. A hydrogen passage 68 branching from the hydrogenrecirculation passage 9 is connected to the electrically heated catalyst67. The other end of the discharge air passage 69, one end of which isconnected to the air passage outlet 14 of the fuel cell 1, is alsoconnected to the electrically heated catalyst 67. A normally-closedcut-off valve 70 is provided in the hydrogen recirculation passage 9. Anormally-closed flow control valve 71 is provided in the branchinghydrogen passage 68. A pressure control valve 72 is provided in thedischarge air passage 69.

The cut-off valve 3 and the pressure control valve 4 in the hydrogensupply passage 6 are opened, the cut-off valve 70 in the hydrogenrecirculation passage 9 is closed and the flow control valve 71 in thebranching hydrogen passage 68 is opened to supply hydrogen. In thissituation, hydrogen in the hydrogen tank 2 is supplied to the catalyticcombustor 66 through the hydrogen supply passage 6, the hydrogen passagein the fuel cell 1 and a branching hydrogen passage 68. When thecompressor 11 is placed in the ON position and the pressure controlvalve 72 in the discharge air passage 69 is opened, air injected fromthe compressor 11 is supplied to the catalytic combustor 66 through theair supply passage 12, the air passage in the fuel cell 1 and thedischarge air passage 69. Hydrogen and oxygen present in the air arecombusted in the catalytic combustor 66. Combustion gas flows into theheat exchanger 65 and heats coolant by heat exchange. Thereafter thecombustion gas is discharged to the outside atmosphere.

Since hydrogen is not reacted (combusted) with oxygen in the air untilthe catalyst in the catalytic combustor 66 reaches an activationtemperature, the electrically heated catalyst 67 increases thetemperature of the hydrogen gas and the air to a temperature enablingignition in the catalytic combustor 66.

Next the flowcharts in FIGS. 2-4 showing startup control of the fuelcell system executed by the controller 51 will be described.

Referring to the flowchart in FIG. 2, a control routine for setting thebelow-freezing startup mode flag will be described. This control routineis executed only once when a signal from a switch 85 is switched fromthe OFF position to the ON position (in other words, when startupoperations are commenced). The switch 85 may comprise a key switchnormally provided in a vehicle. When the switch 85 is switched from OFFto ON to startup the vehicle, the fuel cell system is started.

Firstly in a step S1, the water temperature Ttnk [° C.] in the watertank 31 is read via the temperature sensor 81. In a step S2, the watertemperature Ttnk [° C.] in the water tank 31 is compared with freezingpoint, 0° C. When the water temperature Ttnk [° C.] in the water tank 31is less than or equal to 0° C., it is determined that water in the watertank 31 has frozen and the routine proceeds to a step S3. In the stepS3, the below-freezing startup mode flag is set to a value of unity(below-freezing startup mode flag=1). The below-freezing startup modeflag shows whether the fuel cell system is started at a temperaturebelow or above freezing point. Thereafter the routine proceeds to a stepS4 and a current is applied to the electrically heated catalyst 67. As aresult, the temperature of the electrically heated catalyst 67 isincreased.

At this stage, since it is not possible for the fuel cell 1 to performhigh level power generation, the vehicle can not be operated. Thus in astep S5, the running permission flag is set to zero (running permissionflag=0). The running permission flag is a flag which shows whether ornot the running of the vehicle is prohibited. When the runningpermission flag is set to zero, a lamp near the driver's seat showingthat vehicle running is prohibited is illuminated.

When the water temperature Ttnk in the water tank 31 exceeds freezingpoint 0° C., it is determined that the water in the water tank 31 is notfrozen. In this situation, the supplied gas can be immediatelyhumidified and the system shifts to normal mode. Consequently highlevels of power generation by the fuel cell 1 are enabled immediately.When the routine proceeds from the step S2 to the steps S6, S7, thebelow-freezing startup mode flag is set to a value of zero(below-freezing startup mode flag=0), and the running permission flag isset to unity (running permission flag=1). When the running permissionflag is set to unity, a lamp near to the driver's seat showing thatvehicle running is prohibited is turned off.

Referring to the flowchart in FIG. 3, a first startup control belowfreezing point will be described. The control routine in FIG. 3 isexecuted at a fixed interval (for example 10 msec).

In FIG. 3, in a step S11, the value of the below-freezing startup modeflag is read and it is determined whether the below-freezing startupmode flag is unity or not. When the below-freezing startup mode flag=0,it is not necessary to perform startup control below freezing point.Thus the routine is terminated at that point.

When the below-freezing startup mode flag=1, the routine proceeds to astep S12 where a catalytic combustor ignition flag is read and it isdetermined whether the below-freezing startup mode flag is zero or not.The catalytic combustor ignition flag is a flag showing whether or notit is possible to combust air and hydrogen in the catalytic combustor66. When the catalytic combustor ignition flag=1, it is possible tocombust air and hydrogen in the catalytic combustor 66. The catalyticcombustor ignition flag is set to an initial value of zero when the fuelcell system is started. When the catalytic combustor ignition flag has avalue of zero (catalytic combustor ignition flag=0), the routineproceeds to a step S13 and the temperature Tehc [° C.] in theelectrically heated catalyst 67 is read via the temperature sensor 82.In a step S14, the temperature Tehc [° C.] of the electrically heatedcatalyst 67 is compared with a predetermined temperature T1 [° C.]. Thepredetermined temperature T1 is the minimum temperature (for example,70-80° C.) at which ignition of the gas comprising air and hydrogen ispossible in the catalytic combustor 66.

If the temperature Tehc of the electrically heated catalyst 67 is lessthan a predetermined temperature T1, the routine is terminated. At thistime, a current is applied to the electrically heated catalyst 67 andthe temperature Tehc of the electrically heated catalyst 67 increases.

When the temperature Tehc of the electrically heated catalyst 67 isgreater than or equal to the predetermined temperature T1, the routineproceeds from the step S14 to a step S15 and the catalytic combustorignition flag is set to unity (catalytic combustor ignition flag=1). Atthis time, hydrogen and air flows into the catalytic combustor 66 and iscombusted. In a step S16, the compressor 11 is switched to the ONposition in order to supply air to the catalytic combustor 66 and thepressure control valve 72 is opened. The cut-off valve 3 and thepressure control valve 4 are opened in order to supply hydrogen gas tothe catalytic combustor 66, the cut-off valve 70 is closed and the flowcontrol valve 71 is opened.

In this manner, the air discharged from the compressor 11 passes throughthe fuel cell 1 and the pressure control valve 72 and is supplied to theelectrically heated catalyst 67. Hydrogen flows through the cut-offvalve 3, the pressure control valve 4, the fuel cell 1, the flow controlvalve 71 and is supplied to the electrically heated catalyst 67. Afterbeing heated by the electrically heated catalyst 67, hydrogen gas andair is introduced into the catalytic combustor 66 and hydrogen iscombusted using oxygen in the air by the catalyst which has reached anactivation temperature. The resulting combustion gas is introduced intoa heat exchanger 65 and discharges heat due to heat exchange with thecoolant. Thereafter the combustion gas at reduced temperature isdischarged to the atmosphere. Since the cut-off valve 70 is closed,hydrogen in the hydrogen recirculation passage 9 of the injector 5 isnot recycled.

In the step S17, the coolant which has been heated by combustion gas inthe heat exchanger 65 is transferred to the fuel cell 1 through a firstbypass passage 27 by switching on the coolant pump 21. The controller 21applies a current to the three-way valve 28 so that the port A is cutoff from the port C and the port B is connected to the port C. In thismanner, coolant heated in the heat exchanger 65 flows into the innersection of the fuel cell 1 through the fuel cell inlet 23 via thecoolant passage 22 from the first bypass passage 27. Thereafter coolantwhich has a lower temperature due to heat loss in the fuel cell 1 flowsthrough the coolant passage 25 from the fuel cell outlet 24 and returnsto the heat exchanger 65 of the first bypass passage 27. In a step S18,since combustion in the catalytic combustor 66 has already commenced,the application of current to the electrically heated catalyst 67 isstopped and power consumption by the battery is stopped.

Since the catalytic combustor ignition flag=1, when the control routineis performed on the immediately subsequent occasion, the routineproceeds from the step S12 to the step S19 and the coolant temperatureTout [° C.] in the fuel cell outlet 24 is read as a measure of thetemperature of the fuel cell 1 via the temperature sensor 83. In a stepS20, the coolant temperature Tout [° C.] in the fuel cell outlet 24 iscompared with freezing point, 0° C.

If the coolant temperature Tout [° C.] in the fuel cell outlet 24 isless than freezing point 0° C., the routine proceeds to a step S21. Inthe step S21, the hydrogen flow amount flowing through the flow controlvalve 71 is controlled so that actual hydrogen flow amount detected bythe flow amount sensor 52 coincides with the reference hydrogen flowamount. The reference hydrogen flow amount is determined in advance sothat heat generation is performed efficiently in the catalytic combustor66. In this manner, the heat release amount of the catalytic combustorbecomes a fixed value for example (refer to the solid line in FIG. 5A).

FIG. 5 is a schematic diagram of variation over time of the catalyticcombustor heat release value after the fuel cell 1 is started at atiming t1 when water in the water tank 1 is below freezing point (thesolid line in FIG. 5A), of the power generation amount in the fuel cell1 (the broken line in FIG. 5A), of the coolant temperature at the fuelcell outlet (FIG. 5B) and of the ice melt amount (FIG. 5C).

In a step S22, the compressor 11 and the pressure control valve 72 arecontrolled in order to regulate the pressure and the flow amount of theair discharged from the compressor 11 so that the actual combustiontemperature detected by the temperature sensor 84 coincides with areference temperature. When the actual combustion temperature is lowerthan the reference temperature, the air-fuel ratio (the ratio of thehydrogen flow amount and the air flow amount) is controlled to be richby the decrease in the air flow amount introduced to the catalyticcombustor 66. In this manner, the combustion temperature increases.Conversely when the actual combustion temperature is higher than thereference temperature, the air-fuel ratio is controlled to be lean bythe increase in the air flow amount. In this manner, the combustiontemperature decreases.

The coolant pump 21 is operated and the heat generated by the catalyticcombustor 66 is applied to the coolant by the heat exchanger 65 whichresults in a temperature increase in the coolant. The coolant which hasundergone a temperature increase passes through the three-way valve 28from the first bypass passage 27 and flows into fuel cell 1. The heatgenerated in the catalytic combustor 66 is transferred to the fuel cell1 through the recirculation of the coolant and allows the temperature ofthe fuel cell 1 to increase. The fuel cell 1 normally has the coolantpassage 95 arranged such that the fuel cell 1 performs extremelyefficient heat exchange operations with the coolant flowing through thecoolant passage 95 in the fuel cell 1. Thus the temperature of thecoolant at the coolant outlet 24 of the fuel cell 1 falls to atemperature which is substantially equal to the temperature of the fuelcell 1. Thus although the fuel cell 1 undergoes a temperature increasedue to the absorbed heat, the temperature of the coolant at the fuelcell outlet 24 is substantially equal to the temperature of the fuelcell 1. Thus if the fuel cell 1 is to be greater than or equal to 0° C.,the coolant temperature at the fuel cell outlet 24 must also be greaterthan or equal to freezing point 0° C. Even when high levels of powergeneration are performed if the fuel cell 1 is below freezing point (forexample—20° C.), water produced as a result of power generation iscooled and refreezes in the fuel cell 1 which is below freezing point.If the fuel cell 1 is not heated to a temperature at which water doesnot refreeze, high levels of power generation in the fuel cell 1 are notpossible. Thus high levels of power generation are only possible and therunning permission flag allowing vehicle running is set to unity afterthe coolant temperature of the fuel cell outlet 24 exceeds freezingpoint 0° C. (after the time t2 in FIG. 5).

When coolant at a temperature of less than 0° C. flows into the watertank 31 which is downstream of the fuel cell 1, heat is applied to theice gradually by the heat exchanger 61 in the water tank 31. Howeverthis does not result in melting of ice. The freezing point of thecoolant (for example −20° C.) is much lower than 0° C. and it can flowat a temperature less than 0° C. Thus as shown in FIG. 5C, the meltedamount of ice, in other words, the heat used to melt ice issubstantially zero, in this stage. In the heat exchanger 61 in the watertank 1, the coolant does not lose much heat due to a lack of heat offusion applied to ice. In this manner, it is possible to apply heatproduced in the catalytic combustor 66 efficiently only to the fuel cell1. Thus the heat can be applied to the fuel cell 1 which is at atemperature in the vicinity of 0° C. With respect to the same hydrogenconsumption amount, it is possible to rapidly reach high levels of powergeneration by the fuel cell 1 and a state enabling vehicle running.

In this manner, when the coolant temperature Tout at the fuel celloutlet 24 is greater than or equal to 0° C., it is determined that thefuel cell 1 is in a condition enabling high levels of power generation.Thus the routine proceeds from the step S20 to the step S23 and S24. Asa result, below-freezing startup mode is terminated and vehicle runningis permitted. In a step S23 the below-freezing startup mode flag is setto zero and in a step S24 the running permission flag is set to unity.When the running permission flag has a value of unity, a lamp near tothe driver's seat showing that vehicle running is prohibited is turnedoff.

When the driver operates the vehicle after the lamp is turned off, thefuel cell 1 generates the electrical power required for vehicleoperation. Therefore, in a step S25, the cut-off valve 70 is opened, theflow control valve 71 is closed, hydrogen supply to the catalyticcombustor 66 is terminated and hydrogen is recirculated to the hydrogenrecirculation passage 9. In other words, the controller 51 stops theoperation of the heat exchanger 65 (heater) by stopping supply ofhydrogen to the catalytic combustor 66. This is due to the fact that itis possible to heat the coolant flowing into the fuel cell 1 using thepower generated in the fuel cell 1. In this manner, combustion in thecatalytic combustor 66 is finished and the combustor heat release valuebecomes zero (the solid line in FIG. 5A).

However at this time (the time t2 in FIG. 5), ice in the water tank 1 isnot melted. As a result, in the step S26, the defrosting flag (initialvalue of zero at the beginning of the startup operation of the fuel cellsystem) is set to unity (defrosting flag=1). The defrosting flag is aflag showing whether or not melting of ice is currently performed in thewater tank 31. When it is assumed that the vehicle is operatedimmediately after the lamp showing that vehicle running is prohibited isturned off, the power generation amount in the fuel cell 1 increasesafter the time t2 as shown by the broken line in FIG. 5A.

When the vehicle is running, the temperature of the fuel cell 1increases as a result of power generation in the fuel cell 1, andcoolant flowing in the fuel cell 1 is heated by heat (waste heat)produced by the fuel cell 1. As a result, as shown by the solid line inFIG. 5B, the coolant temperature Tout of the fuel cell outlet 24increases to a temperature which is higher than 0° C. Heat(corresponding to heat of fusion) is applied to ice by the heatexchanger 61 in the water tank 31 as a result of the circulation ofcoolant which is at a temperature higher than 0° C. As a result, ice ismelted. The melted amount of ice which is zero at a time t2 as shown inFIG. 5C increases as a function of time. At a time t4, melting of all ofthe ice in the water tank 31 is finished and the amount of ice meltedafter the time t4 becomes fixed. After the time t4, the coolanttemperature at the fuel cell outlet 24 increases more rapidly thanbefore the time t4.

In this manner, in this embodiment, firstly the fuel cell 1 is heated togreater than or equal to a value of 0° C. using coolant which has a hightemperature as a result of heat exchange in the heat exchanger 65.Melting of ice in the water tank is performed using the coolant appliedheat (waste heat) produced during power generation in the fuel cell.After melting of all the ice in the water tank 31 is finished, the fuelcell system shifts to normal operation mode.

FIG. 4 describes a second startup control below freezing point. Thecontrol routine in FIG. 4 is executed at a fixed period (for example, 10msec).

In a step S31 in FIG. 4, the running permission flag is read todetermine whether or not the running permission flag is unity, and in astep S32 the defrosting flag is read to determine whether or not thedefrosting flag is unity. When the running permission flag=1 and thedefrosting flag=1, the routine proceeds to the step S33 where a coolanttemperature Tout [° C.] at the fuel cell outlet is read via thetemperature sensor 83. In the step S34, the coolant temperature Tout [°C.] at the fuel cell outlet is compared with a second predeterminedtemperature T2 [° C.].

The second predetermined temperature T2 is the temperature when meltingof all the ice in the water tank 31 should be finished. The secondpredetermined temperature T2 depends on the power generation amount inthe fuel cell 1. The ROM of the controller 51 may store a table ofpredetermined temperatures T2 corresponding to a power generation amountin the fuel cell 1. Thus the controller 51 may look up a table in orderto calculate a predetermined temperature T2 from an actually detectedpower generation amount of the fuel cell. Put simply, a predeterminedtemperature T2 may be a constant. In a step S34, other than thetemperature comparison, it may be determined whether or not the meltingof all the ice in the water tank 31 is finished based on the powergeneration amount of the fuel cell 1.

When the coolant temperature Tout of the fuel cell outlet is less thanthe fixed temperature T2, since melting of all ice in the water tank 31is not finished, the routine is terminated. When the coolant temperatureTout of the fuel cell outlet 24 is greater than or equal to thepredetermined temperature T2 (the time t4 in FIG. 5), it is determinedthat melting of all of the ice in the water tank 31 is finished and theroutine proceeds to a step S35. In the step S35, the defrosting flag isset to a value of zero (defrosting flag=0). Thereafter in a step S36,application of current to the three-way valve 28 is stopped, the port Band the port C are cut off and the port A and the port C are connected.In this manner, the fuel cell system shifts to normal mode and coolantflows through the radiator 26.

The effect of this embodiment will be described hereafter.

(a) Since water freezing in the water tank 31 is melted using heat(waste heat) in the fuel cell 1 produced as a result of powergeneration, it is possible to prevent excess consumption of hydrogen gasin the fuel cell 1 or excess use of power. In this manner, fuelconsumption in the fuel cell system can be reduced. Most of the heatproduced below freezing point by power generation can be used toincrease the temperature of the fuel cell 1. Thus it is possible toshorten the time (the startup time of the fuel cell) until high levelsof power generation are enabled.

(b) Since the heat exchanger 61 in the water tank 31 is disposeddownstream of the fuel cell 1, heat in the coolant heated in the heatexchanger 65 is transmitted to the fuel cell upstream of the water tank31 and the temperature increase of the fuel cell 1 can be optimized.After undergoing heat exchange in the fuel cell 1, the coolant flows tothe heat exchanger 61 in the water tank 31. In this case, since thetemperature of the coolant leaving the fuel cell 1 is lower than thetemperature of the main section of the fuel cell 1, it is possible toprevent the temperature of the coolant leaving the fuel cell 1 fromexceeding 0° C. until the temperature of the main body of the fuel cell1 is greater than or equal to 0° C. In this manner, the energy requireduntil high levels of power generation start in the fuel cell 1 is nothigh and the fuel cell 1 can be started rapidly.

(c) During high load operations when the external temperature is high,the fuel cell 1 of course has a high temperature. The first bypasspassage 27 (normally narrow) is adapted so that coolant does not flowunder these conditions. A large amount of coolant flows into the coolantrecirculation passage (22, 25) and the temperature of the fuel cell 1 isreduced. If a heat exchanger 65 was provided in the coolantrecirculation passage (22, 25) as a heater, it would be necessary toincrease the capacity of the coolant pump 21 due to pressure loss.However in this embodiment, since the heat exchanger 65 is provided inthe first bypass passage 27 to act as a heater instead of being providedin the coolant recirculation passage (22, 25), it is possible to avoidan increase in the pressure loss of the coolant recirculation passage(22, 25).

(d) The water tank 31 is positioned on the coolant passage 25 upstreamof the branching position of the first bypass passage 27 from thecoolant passage 25. As a result, water in the water tank 31 can bemaintained at a high temperature by the coolant warmed by the fuel cell1 not only while the fuel cell 1 is warmed up but even when the fuelcell 1 is operated at a high temperature when cooling the fuel cell 1 isnecessary. In this manner, less heat is required to evaporate water usedfor humidifying operations. In particular, when the water tank 31 isinsulated, the effect is increased.

(e) When the temperature of the fuel cell 1 is less than 0° C., thewater in the water tank 31 does not exceed 0° C. As a result, when theheat exchanger 65 is operated as a heater in a region in which thetemperature detected value of the fuel cell is less than 0° C., a largeamount of heat from the heat exchanger 65 is not used as heat of fusionto melt ice. The heat energy of the heat exchanger 65 is used toincrease the temperature of the ice in the water tank 31 to 0° C. Inthis manner, it is possible to melt ice in the water tank 31 using onlywaste heat from the fuel cell 1. Therefore it is possible to suppresshydrogen gas consumption other than the hydrogen gas required for powergeneration.

Referring to FIG. 6 and FIG. 7, a second embodiment will be described.FIG. 6 shows a schematic figure of a fuel cell system according to asecond embodiment. The flowchart shown in FIG. 7 shows a startup controlroutine below freezing point according to the second embodiment. FIG. 6replaces FIG. 1 and FIG. 7 replaces FIG. 3. In FIG. 6, the samecomponents are designed by the same reference numerals as those used inFIG. 1. In FIG. 7, the same step numbers are designed by the samereference numerals as those used in FIG. 3.

As shown in FIG. 6, the point of difference of the second embodimentfrom the first embodiment is that the fuel cell system is provided witha second bypass passage 91 which bypasses the water tank 31 and isconnected from the coolant passage outlet 24 of the fuel cell 1 to thefirst bypass passage 27. The second bypass passage 91 is connected tothe bypass passage 27 through the three-way valve 92. When no current isapplied, the three-way valve 92 connects the port D with the port F andcuts off the port E from the port F. When current is applied, the port Dis cut off from the port F and the port E is connected to the port F.

In FIG. 7, the point of difference from the first embodiment lies in theaddition of the step S41. In the FIG. 7, when the temperature Tehc ofthe electrically heated catalyst 67 reaches a predetermined temperatureT1 and enables ignition in the catalytic combustor 66, supply of air andhydrogen to the catalytic combustor 66 is performed in the step S16 andS17. In the first embodiment, the coolant flows through the heatexchanger 61 in the water tank 31. In contrast in the second embodiment,coolant bypasses the water tank 31 since a current is not applied to thethree-way valve 92 in this stage.

Thereafter the coolant temperature Tout of the fuel cell outlet 24 takesa value of greater than or equal to 0° C. Thus high levels of powergeneration by the fuel cell 1 are possible and vehicle operation ispermitted. In the step S25, hydrogen supply to the catalytic combustor66 is stopped and the routine is proceeds to a step S41. In the stepS41, the controller 51 applies a current to the three-way valve 92 sothat coolant flows via the heat exchanger 61. In other words, thecontroller 51 commands the three-way valve 92 to cut off the connectionof the port D and the port F and connect the port E with the port F.

Thus the point of difference from the first embodiment of the secondembodiment is that while the coolant temperature Tout (which can berepresentative of the temperature of the fuel cell) in the fuel celloutlet 24 is less than 0° C., coolant flows through the second bypasspassage 91 to the three-way valve 92 (passage switching passage) anddoes not flow via the heat exchanger 61. In this manner, heat energyproduced in the heat exchanger 65 (the heater) in the water tank 31 isnot used in the heat exchanger 61 in the water tank 31. In other words,the heat exchanger 65 does not generate heat energy for increasing thetemperature of ice and latent heat of fusion required for melting ice inthe water tank 31. In this manner, it is possible to shorten the startuptime of the fuel cell 1.

When the coolant temperature Tout of the fuel cell outlet 24 takes avalue which is greater or equal to a second predetermined temperature T2and melting of all ice in the water tank 31 is finished, a current isnot applied to the three-way valve 28, and thus coolant is recirculatedthrough the radiator 26 and the heat exchanger 61.

The above two embodiments rest on the premise that the heat exchangecharacteristics with the coolant in the fuel cell are extremelyefficient and that the temperature of the fuel cell is approximatelyequal to the coolant temperature Tout at the outlet of the fuel cell 1.However when heat exchange characteristics with the coolant in the fuelcell are poor and the coolant temperature Tout at the outlet of the fuelcell 1 diverges from the fuel cell temperature, the temperature sensor83 performs direct detection of the temperature of the main body of thefuel cell 1 instead of detecting the temperature of the coolant at thefuel cell outlet. Thus the temperature at which power generation by thefuel cell 1 is enabled can be accurately detected.

Further, hydrogen supplied to the fuel cell 1 in the above twoembodiments may comprise hydrogen (reformate gas) obtained as a resultof reforming a hydrocarbon fuel.

The entire contents of Japanese Patent Application P2002-301448 (filedOct. 16, 2002) are incorporated herein by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

1. A fuel cell system having: a fuel cell generating power as a resultof chemical reactions between supplied gases, wherein a coolant flows inthe fuel cell and undergoes a temperature increase as a result ofabsorbing waste heat produced by power generation in the fuel cell; awater tank; a humidifying device for humidifying at least one suppliedgas by using water from the water tank; and a coolant temperatureregulation device for regulating a temperature of the coolant flowinginside the fuel cell so as to control the temperature of the fuel cell;the fuel cell system comprising: a defrosting device for melting ice inthe water tank by applying heat of the coolant to the water tank,wherein the defrosting device is disposed in the water tank andcomprises a heat exchanger allowing heat exchange between the coolantfrom the fuel cell and the ice in the water tank; a heater for heatingthe coolant discharged from the defrosting device; a coolantrecirculation passage for allowing a recirculation of the coolantthrough the defrosting device, the heater and the fuel cell; a flowgenerator for generating a flow of the coolant heated by the heater fromthe heater to the defrosting device via the fuel cell; and a controllerfor controlling a startup operation of the fuel cell system, thecontroller having the function of controlling the flow generator togenerate a flow of coolant from the fuel cell to the defrosting deviceso as to melt ice in the water tank while the startup operation of thefuel cell system.
 2. The fuel cell system as defined by claim 1, furthercomprising a temperature sensor for detecting a temperature of thecoolant; wherein the coolant temperature regulation device comprises: aradiator provided on the coolant recirculation passage; a first bypasspassage branching from the coolant recirculation passage and bypassingthe radiator, the heater being disposed in the first bypass passage; anda passage switching device for selectively switching the passage for thecoolant between a passage passing through the radiator and a passagepassing through the first bypass passage; and wherein the controllerfurther functions to control the passage switching device in response toa detected temperature of the coolant so as to regulate the temperatureof the coolant.
 3. The fuel cell system as defined by claim 2, whereinthe water tank is disposed in the coolant recirculation passage upstreamof the position at which the first bypass passage branches from therecirculation passage.
 4. The fuel cell system as defined by claim 2,wherein the controller functions to control the passage switching deviceto allow the passage for the coolant passing through the radiator whenthe detected temperature of the coolant is greater than or equal to asecond predetermined temperature.
 5. The fuel cell system as defined byclaim 2, further comprising a temperature sensor for detecting a watertemperature in the water tank; wherein the controller is operable tostart the heater when the detected water temperature in the water tankis less than or equal to freezing point of water.
 6. The fuel cellsystem as defined by claim 5, wherein the heater is a heat exchangerintegrated with a catalytic combustor and an electrically heatedcatalyst for increasing the temperature of gases supplied to thecatalytic combustor, the heat generated by the catalytic combustor beingapplied to the coolant by the heat exchanger, wherein the controllerfunctions to start the heater by applying current to the electricallyheated catalyst when the detected water temperature in the water tank isless than or equal to freezing point of water.
 7. The fuel cell systemas defined by claim 6, further comprising a temperature sensor fordetecting a temperature in the electrically heated catalyst; wherein thecontroller functions to start the flow generator and to allow a passageof the coolant through the first bypass passage, when the detectedtemperature in the electrically heated catalyst is greater than or equalto a first predetermined temperature.
 8. The fuel cell system as definedby claim 1, further comprising a temperature sensor for detecting atemperature of the fuel cell; wherein the controller further functionsto compare the detected temperature of the fuel cell with freezing pointof water; operate the heater when the detected temperature of the fuelcell is less than freezing point; and stop the operation of the heaterwhen the detected temperature of the fuel cell is greater than or equalto freezing point.
 9. The fuel cell system as defined by claim 1,further comprising: a second bypass passage branching upstream of thewater tank and bypassing the water tank; and a passage switching devicefor switching the passage for the coolant between a passage passingthrough the heat exchanger in the water tank and a passage passingthrough a second bypass passage.
 10. The fuel cell system as defined byclaim 9, further comprising a temperature sensor for detecting atemperature of the fuel cell, wherein the controller further functionsto compare the detected temperature of the fuel cell with freezing pointof water; control the passage switching device so that the coolant flowsthrough the second bypass passage when the detected temperature of thefuel cell is less than freezing point; and control the passage switchingdevice so that the coolant flows through the heat exchanger in the watertank when the detected temperature of the fuel cell is greater than orequal to freezing point.
 11. A vehicle comprising the fuel cell systemas defined by claim
 1. 12. A fuel cell system having: a fuel cellgenerating power as a result of chemical reactions between suppliedgases, wherein a coolant flows in the fuel cell and undergoes atemperature increase as a result of absorbing waste heat produced bypower generation in the fuel cell; a water tank; a humidifying devicefor humidifying at least one supplied gas by using water from the watertank; and a coolant temperature regulation device for regulating atemperature of the coolant flowing inside the fuel cell so as to controlthe temperature of the fuel cell; the fuel cell system comprising: adefrosting means for melting ice in the water tank by applying heat ofthe coolant to the water tank, wherein the defrosting means is disposedin the water tank and comprises a heat exchanger means allowing heatexchange between the coolant from the fuel cell and the ice in the watertank; a heater means for heating the coolant discharged from thedefrosting means; a coolant recirculation passage means for allowing arecirculation of the coolant through the defrosting means, the heatermeans and the fuel cell; a flow generating means for generating a flowof the coolant heated by the heater mans from the heater means to thedefrosting means via the fuel cell; and a control means for controllingthe flow generator to generate a flow of coolant from the fuel cell tothe defrosting means so as to melt ice in the water tank while a startupoperation of the fuel cell system.
 13. A control method for controllinga fuel cell system, the fuel cell system having: a fuel cell generatingpower as a result of chemical reactions between supplied gases, whereina coolant flows in the fuel cell and undergoes a temperature increase asa result of absorbing waste heat produced by power generation in thefuel cell; a water tank; a humidifying device for humidifying at leastone supplied gas by using water from the water tank; and a coolanttemperature regulation device for regulating a temperature of thecoolant flowing inside the fuel cell so as to control the temperature ofthe fuel cell; the control method comprising the steps of: providing adefrosting device for melting ice in the water tank by applying heat ofthe coolant to the water tank, wherein the defrosting device is disposedin the water tank and comprises a heat exchanger allowing heat exchangebetween the coolant from the fuel cell and ice in the water tank;providing a heater for heating the coolant discharged from thedefrosting device; providing a coolant recirculation passage forallowing a recirculation of the coolant through the defrosting device,the heater and the fuel cell; and generating a flow of coolant heated bythe heater from the heater to the defrosting device via the fuel cell soas to melt ice in the water tank while a startup operation of the fuelcell system.